If you are using default Visual Studio project template, the ConfigureAuth method could be found in partial class Startup.Auth.cs. So make sure you didn't break anything when modifying project structure.

This is an example of ConfigureAuth method:

// For more information on configuring authentication, please visit http://go.microsoft.com/fwlink/?LinkId=301864
public void ConfigureAuth(IAppBuilder app)
{
// Configure the db context and user manager to use a single instance per request
app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);
// Enable the application to use a cookie to store information for the signed in user
// and to use a cookie to temporarily store information about a user logging in with a third party login provider
app.UseCookieAuthentication(new CookieAuthenticationOptions());
app.UseExternalSignInCookie(DefaultAuthenticationTypes.ExternalCookie);
// Configure the application for OAuth based flow
PublicClientId = "self";
OAuthOptions = new OAuthAuthorizationServerOptions
{
TokenEndpointPath = new PathString("/api/Token"),
Provider = new ApplicationOAuthProvider(PublicClientId),
AuthorizeEndpointPath = new PathString("/api/Account/ExternalLogin"),
AccessTokenExpireTimeSpan = TimeSpan.FromDays(14),
AllowInsecureHttp = true
};
// Enable the application to use bearer tokens to authenticate users
app.UseOAuthBearerTokens(OAuthOptions);
}

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